1. Field of the Invention
This invention relates to art of improving the reliability of semiconductor lasers, and the stability of the laser output, mainly to a multi-beam semiconductor laser including a plurality of light emitting regions which are formed on one and the same semiconductor substrate and can be driven independently from each other.
2. Related Background Art
Presently semiconductor lasers are widely used in laser printers and optical disk memory systems, but writing rates of the printers and data transmission rates of the optical disks are not always sufficiently high. In such circumstances, it is expected that if it is possible to neighbor two laser beam emitting regions to each other independently operative on one and the same semiconductor laser substrate, then systems having twice writing rates or data transmission rates with one optical system will be realized. To this end, recently such multi-beam semiconductor lasers have been studied. Lasers which are currently used in such applications are AlGaAs-based and have wavelengths of 0.78.about.0.84 .mu.m. As the art for the electrical isolation of the beam emitting regions in lasers of this material are known diffusion of impurities between the beam emitting regions for higher resistances, etching recesses deeper than the active layers in the steps of forming the beam emitting regions and burying the etched recesses with low-refractive index higher resistance crystals, and so on.
On the other hand, AlGaInP-based infrared semiconductor lasers of wavelengths of 0.6.about.0.7 .mu.m have been developed. The replacement of AlGaAs-based semiconductor lasers with AlGaInP-based semiconductor lasers enables sensitive materials of high sensitivity to be used in laser printers, with a result that higher printing rates can be expected. In optical disks, it will be possible to more focus beam spots, with a result of larger capacities.
But the multi-beam art, which has been much studied on AlGaAs-based lasers, has not been much developed on AlGaInP-based lasers. That is, the application of the art of diffusing impurities between beam emitting regions for higher resistances or the art of etching recesses deeper than the active layers and burying the recesses with low-refractive index high-resistance crystals, which are multi-beam art for AlGaAs-based semiconductor lasers, to AlGaInP-based semiconductor lasers as it is does not produce sufficient characteristics of multi-beam semiconductor lasers in which the respective beams can be driven independently of each other. The art in which respective beams are driven not independently of each other but concurrently simply for higher outputs is known ("High-Power AlGaInP Three-Ridge Type LASER Diode Array", Electronics Letters, 17th, 1988, Vol. 24, No. 6).
The multi-beam semiconductor laser includes, on one laser chip, a plurality of laser resonators of independently driven type each including an oscillation region and a drive unit. When the multi-beam semiconductor laser is actuated, the drive units of required ones of the laser resonators are selected and each fed with a required voltage, and the respective selected drive units supply operating currents above a threshold current, so that the oscillation regions of the required laser resonators are driven to emit laser beams. These laser beams are led to light detecting mediums through optical systems, optical fibers, etc.
In driving laser resonators, an output level of the laser beams is high immediately after an actuation, and the output level lowers as time passes. This is because, since none of the elements generate heat before the actuation, the oscillation regions can be easily excited to emit beams at a low threshold current, but, after the emission of the beams, emission heat of a set calorie is generated and raises a temperature of the laser chips, with the result that a higher current is required to set on the laser oscillation. A percentage of drop of a laser oscillation from an actuation of an Oscillation is called "droop ratio".
In the AlGaAs multi-beam semiconductor laser, because of high operating currents and relatively high heat resistance, heat conduction is apt to occur between the laser resonators. This results in large fluctuations of output levels of the beams emitted from the respective laser resonators, and low operational reliability of the semiconductor laser. To prevent the influence of the heat crosstalks, the laser resonators have to be formed spaced by a large distance from each other, a heat conduction-hindering member has to be provided therebetween, or other means are necessary. This has been a neck to the reduction of its fabrication costs.
The above-described heat crosstalks can be improved by improving the heat radiation of the chip. One of its methods is described in "The 1991 Autumn Convention of Applied Physics, 11p-ZM-18, Murata et al." This method will be briefed. The electrodes of a multi-beam laser are printed on a Si wafer to form an electrode board, and a chip with a multi-beam semiconductor laser fabricated on is die-bonded epidown (with the substrate up) to the electrode board. A radiator called heat-pass wire is provided on the side of the substrate. The provision of the heat-pass wire improves the heat radiation. But the heat crosstalks cannot be sufficiently suppressed.
The described-above means is effective for the prevention of changes of an output level of laser beams of the semiconductor lasers in continuous operations but does not work sufficiently to suppress changes of an output level of the laser beams immediately after the start of a drive.
Especially in the case that a plurality of laser beams are used, when one of the laser resonators is intermittently driven while the other laser resonator is being driven, an output level of the laser beams being emitted greatly changes due to heat changes and the interference caused by the heat changes.
This results in problems that in, e.g., laser printers, static latent images to be record patterns have disuniform densities, and images or letters have disuniform densities, and other problems.
In the case that a plurality of laser beams are used as a light emitting region of an optical disk, recording/reading of the memory of the optical disk cannot be correctly performed. In the case that a plurality of laser beams are used as a signal light emitting region of the optical communication, this results in the problem that changes of a signal level cause errors in demodulating data, and other problems.
The semiconductor lasers used in laser printers, optical disk memory systems and so on, have different lifetimes depending on their use conditions and environmental temperatures. Especially multi-beam semiconductor lasers generally have shorter lifetimes because of their disuniform characteristics of the respective laser beams. As a countermeasure, photo-diodes or others for the detection of the optical outputs are disposed near the semiconductor lasers for constantly monitoring levels of the outputs to confirm longevities of the semiconductor lasers and the prevention of system troubles.
But it has been very difficult to monitor output levels of individual laser beams because an interval between the laser beams is as small as 10 & 100 .mu.m. Irrespective of single- or multi-beam semiconductor lasers, it is bothering to mount photo-diodes near the semiconductor lasers. Improvements have been expected. Furthermore, the monitoring takes into account actual temperatures of the semiconductor chips, sometimes it is difficult to judge longevities of the semiconductor lasers.
But, semiconductor lasers have different lifetimes depending on frequencies of their drives, their output levels, their environmental temperatures, etc., and it is impossible to exactly estimate their lifetimes. Factually their lifetimes cannot be told until they are put to actual uses. Sometimes, of the components of a system a semiconductor laser is least reliable. In such case, the reliability of the semiconductor laser determines the reliability of the system.
In such circumstances, conventionally means for detecting an output drop of a laser beam or means for detecting a threshold current increase are provided on the system, so that when a detected value exceeds a set allowable range, the termination of a lifetime of a semiconductor laser is judged, and the abnormality is indicated. At this time an operator displaces the abnormal semiconductor laser with a normal semiconductor laser.
But the system includes an optical system which is adjusted with high precision. Usually a new semiconductor laser has to be well positioned with respect to the optical system. It is a problem that this operation takes much time. Furthermore, drops of an output level of laser beams and increases of a threshold current are not always due to the termination of a lifetime of the semiconductor laser. For example, the same phenomenon takes place due to temperature changes caused by thermal crosstalks. As the result, because of the erroneous detection of the termination of the lifetime of the semiconductor laser, the unnecessary replacement of the semiconductor laser has been made.
On the other hand, in the case that a system uses parts of low reliability, it is general that supply parts of the same specifications are incorporated for high reliability of the system. But the system using a semiconductor laser, which has the optical system positionally adjusted, has to include an optical system which is exclusive for the supply parts, which makes the system very expensive.